1. Field of the Invention
The invention generally relates to the recovery of nutrients during the treatment of lipid extracted algae waste to particular, the invention provides an economical, integrated system for the recovery of nutrients and other by-products from hydrolysis and digestion of lipid extracted algae waste and for their re-use in algae growth or other systems.
2. Background of the Invention
With many technical and resource challenges remaining, there is a general consensus that algae have the most potential for significant portion of crude oil displacement among all the known biomass. According to Chisti (Biodiesel from Microalgae, Biotechnology Advances. 25 (2007):294-306), it takes approximately 1.1% and 2.5% of existing US cropping area to meet 50% of all transportation fuel needs of the United States assuming 70% and 30% of oil in dry algae biomass, respectively. If used as a means for CO2 sequestration, 1.6-1.8 lb of CO2 will be fixed for each lb of algae biomass produced. Consuming each lb of algae-based biofuel will reduce 3 lbs of CO2 release associated with burning the same amount of fossil fuel. According to the Department of Energy “Roadmap”, commercial algal growth processing could result in the co-generation of about 190 million tons of lipid-extracted biomass per year if the diesel consumption of the US were replaced by algal lipid based biofuel. Therefore, developing a feasible technology for converting the biomass to a product that has great demand is critical for the establishment of this industry.
Due to anticipated large amount of biomass associated with algae fuel production, there are most likely several options for the utilization of the lipid-extracted spent biomass. The first one is direct conversion of the biomass after oil extraction into biogas through the process known as anaerobic digestion (AD), a process in which complex organic materials are biologically degraded into mainly methane, CO2, ammonia nitrogen (N), inorganic phosphorous (P) and other minor components. The second one is direct gasification/pyrolysis of the biomass into syngas or bio-oil; the third one is to use the biomass as animal feed; and the last one is to fractionate and separate the polysaccharides of the cell walls (mainly cellulose) for further hydrolysis into fermentable sugar to be used to produce fuel such as ethanol or hydrocarbons. There are other options such using the spent biomass for animal feed and as raw materials to produce so-products for industrial purposes. Such options requires further developments of special technologies for proper separation of algae lipid from the rest of the algal biomass.
Among all these options, AD appears to be the most feasible. First, AD is a relatively low-cost technology that is well suited for treating wet organic materials. Second, AD can facilitate recovery of nutrients, principally nitrogen and phosphorous whose sustainable supply is critical to the success of large-scale algal fuel production. Recovering nutrients for recycling has great importance as growing the required algae biomass needs a tremendous amount of fertilizers. Manufacturing N fertilizer is very energy intensive. Additionally, P fertilizer is a non-renewable resource that has finite supply. AD technology fits well within these requirements and limitations and brings multiple benefits as it allows for total recovery of the energy and materials and minimizing the input to the system. In the scenario when the residue biomass is used as animal feed, the same AD process can be employed at the animal farm to recover the carbon and nutrients from the animal manure.
Although AD as a process for organic waste stabilization has been widely used, none of the existing AD technologies is adequate for dealing with the waste that is produced nor has a systematic integrated approach to introducing the AD process into the particularities of an algae growth process been developed. Many in the industry simply attempt to introduce the AD effluent directly into the algal growth systems. However, direct inclusion of the colored, colloidal, pathogen contaminated effluent is quite inappropriate for use in an algal growth system as it results in considerable photo-inhibition and contamination, and requires considerable resources for handling the water in the effluent. On the other hand, recognition and resolution of these concerns for secondary treatment with traditional wastewater treatment processes can be very costly. What is required is a transformative technology that can overcome the described concerns present within AD effluent while reducing cost inputs through effective system design and integration.